This invention relates to determination of density of substances in gaseous form.
It is important to know the density of a gas in many industries, in particular, in the area of petroleum and petrochemical processing. A typical application is a mass flow meter, where volumetric flow rate is combined with the density of the flowing stream to produce mass flow rate. One seeking to measure density, particularly on a continuous on-line basis, has a limited choice of apparatus. One commercially available density meter utilizes an oscillating element in the fluid whose density is measured. Oscillation is caused by an electromagnetic field. The frequency of oscillation depends on the density of the fluid. The sensing element is contained in a housing having one-inch flanges for installation in a pipeline. A standard reference, Process Instruments and Controls Handbook, 2nd ed., 1974, edited by Considine, lists only three techniques for measuring density, none of which are well suited for use outside the laboratory. The listed methods (p. 6-152) are as follows.
In a gas specific gravity balance, a tall column of gas is measured by a floating bottom fitted to the gas containment vessel. A mechanical linkage displays movement of the bottom on a scale. A buoyancy gas balance consists of a vessel containing a displacer mounted on a balance beam and with a manometer connected to it. Displacer balance is established with the vessel filled with air and then filled with gas, the pressure required to do so being noted from the manometer in both cases. The pressure ratio is the density of the gas relative to air. In a viscous drag density instrument, an air stream and a stream of the gas under test are passed through separate identical chambers, each containing a rotating impeller. The two streams are acted upon by the rotating impellers and in turn each acts upon a non-rotating impeller mounted in the opposite end of the chamber. The non-rotating impellers are coupled together by a linkage and measure the relative drag shown by the tendency of the impellers to rotate, which is a function of relative density.